Auxiliary Power Engine manufacturers are required to demonstrate by test that the auxiliary rotor cases are able to contain damage caused by the failure of high energy rotor and blades. It is known that a “worst-case” rotor failure is defined if the rotor breaks into three equal weight pieces. This is referred to a tri-hub failure. The containment structure/case around a rotor, for instance, must be strong enough to absorb the energy of the three parts when it breaks apart during such a test.
To test containment structures, first a rotor, in this case an impeller is deliberately slotted in such a way to fail into three pieces when rotated to specified speed. This impeller is then placed into an engine and the engine is operated at it maximum attainable speed until the impeller fails, breaking into three pieces.